In industry and commerce there is need for metals of good hardness with good conductivity. These two properties are quite incongruous since good conductivity is a property of pure metals whereas good hardness is normally achieved by alloying the pure metal with one or more metals.
Copper and silver are the two metals that exhibit the highest electrical and thermal conductivity. Silver has excellent conductivity, but is soft and very expensive. Copper, although relatively expensive, is widely used where high conductivity is necessary and is, in fact, the standard used in rating the conductivity of other metals. However, copper is comparatively soft in its pure state, and to strengthen copper and increase its hardness, it must either be cold worked or alloying elements must be added. Cold working does not reduce the conductivity, but if the application is such that the copper is subsequently heated, the properties obtained by cold working can be lost. Adding alloying elements to copper reduces the conductivity to significantly low levels, depending upon the specific element and quantity used.
Brasses and bronzes, of which there are many kinds, are copper base alloys to which, singly or in combination, such elements as tin, zinc, aluminum, iron, etc., have been added for strength. Such additions seriously reduce the electrical and thermal conductivity. For example, when added singly to pure copper, as little as 0.1% of nickel, aluminum or tin will reduce the 100% electrical conductivity of pure copper to 94%, 91% and 99%, respectively, and a 1% addition of these elements will drop the conductivity to less than 50%. As little as 0.1% silicon or phosphorous will reduce the electrical conductivity of copper at least 50%, with little or no significant improvement in strength or hardness.
Certain elements have a varying degree of solid solubility in copper, which changes with temperature. This makes possible the well known age or precipitation hardened alloys.
Corson U.S. Pat. No. 1,658,186 was an early pioneer in the discovery of the age or precipitation hardening phenomenon in copper base alloys. The basic concept was one in which he found that certain elements could be put into solid solution in selected copper alloys by heating the metal to an elevated temperature, followed by rapid cooling in a quenching media. Then, by reheating to a selected lower temperature for various periods of time, he found that specific metallic compounds could be precipitated out of the solid solution. The effect of this treatment served two purposes. First, the alloying elements precipitated out of solid solution are in the form of discrete particles, which increased the strength and hardness by interfering with the normal mode of physical deformation of the metal under stress. Secondly, it increased the electrical conductivity of the alloy through the effective removal of alloying elements which were precipitated from the copper matrix.
More particularly, the patent to Corson U.S. Pat. No. 1,658,186 describes copper alloys containing silicon, and one or more of a group of silicide forming elements, specifically chromium, cobalt and nickel. In accordance with the Corson invention, improved hardness is achieved by a heat treatment consisting of heating the alloys to a temperature in the range of 705.degree. C. to 975.degree. C. (1382.degree. F. to 1787.degree. F.) and subsequently quenching the alloy to hold the bulk of the alloying elements in solid solution. After quenching, the Corson alloys are aged at a temperature in the range of 250.degree. C. to 600.degree. C. (482.degree. F. to 1112.degree. F.) to precipitate the metallic silicides resulting in an increase in hardness with improvement in electrical conductivity.
As described in the Corson patent, several classes of alloys were produced, including (1) an alloy having an electrical conductivity of 35% and a hardness of 150 Brinell; (2) an alloy of 55% conductivity with a minimum hardness of 135 Brinell; and (3) an alloy with 75% conductivity with a minimum hardness of 110 Brinell. The Corson alloys have never achieved significant commercial importance where both high hardness and high conductivity are required, as in the case of resistance welding electrodes.
In resistance welding of metals, the spot welding tips or contact materials must have good hardness and strength to hold their shape and they must be able to conduct sufficient electrical current to make the weld without undue heating of the contact material, which would cause softening and deformation.
The one common alloy used for resistance welding of stainless steel, is identified by the Resistance Welding Manufacturing Association as the Class 3 type. The specification for this alloy calls for a minimum electrical conductivity of 45% of the conductivity of pure copper, with a minimum hardness of 90 Rockwell B (185 Brinell). The alloy that has been commonly used contains beryllium, the vapors of which have been identified as being toxic. The copper-beryllium alloy must be melted only under the strictest vapor control, and fine grinding dust must be completely collected in the work area. These restrictions have reduced the number of suppliers, and greatly increased the production cost.